LRB Sufficient & Necessary Question Problem Set

[Stephanie Oswalt]

Below are two questions from a student. An instructor will respond below. Thanks!

2017 LR Bible Pg 222 – 224 Sufficient & Necessary Question Problem Set

Question 1. I diagrammed this conditional stimulus with 2 necessary conditions:

EG CC (to mean consumer confidence) + CS (to mean “Consumer skepticism)
-CC or -CS -EG

The correct answer is D, diagrammed as EEG CCB (to mean both consumer confidence and consumer skepticism).

I determined the correct answer of course but my question here is, “Wouldn’t it be more prudent on the LSAT to diagram multiple necessary conditions for this stimulus as part of the prephrasing since you wouldn’t know until you got to answer D that you didn’t need to understand that the necessary condition did not need to be parsed into 2 parts?”

Question 4. I diagrammed this conditional stimulus as:

IMP GS
GS SL
SL MR
IMP GS SL MR
-MR -SL -GS -IMP

Is it best not to create the contrapositive until all elements of the positive chain are in place or should the contrapositive for each element of the chain be created as each element of the positive element is created? Should the contrapositive be created at the end of the chain or as each link of the chain is created?

[Jonathan Evans]

Hi!

Great questions!

Question 1

First, sure, you could parse the necessary condition in the stimulus as two separate necessary conditions. Essentially, both the way the book diagrams this question and your approach are correct. In addition, both approaches are a form of shorthand for this statement. Notice that neither diagram includes the qualifier "a small amount." We could envision a circumstance in which this "small amount" becomes important.

For instance, if we wished to match the principle and an answer choice said "If a nation experiences economic growth, a substantial amount of consumer skepticism is required along with consumer confidence." This would not match the principle in the stimulus.

However, in practice, I would not encourage you to parse and diagram the minutia of every statement out. As you noticed in the correct answer, as long as your analysis is fundamentally sound, you are going to get the answer right. If the diagramming itself becomes the focus of your analysis, you might find that it distracts you from just getting down to business and getting to the right answer.

In other words, diagrams are a tool to help you get to the right answer. If you do a slightly more detailed diagram, that's excellent. If you do a slightly less detailed diagram but still get to the right answer, that's also excellent.

In fact, if you find your initial diagram is not detailed enough to get to the right answer, I can pretty much guarantee that a less detailed (but accurate) diagram will get you down to two contender answer choices. At this point you can revise your initial diagram to account for additional small details. We could have done such a revision for this question if the issue of "a small amount" had been important.

Long story short: you're doing it right. If you had done it like the book, this would have been great too. Sounds like you have a strong grasp of conditional reasoning. Keep up the good work!

Question 2

Great job with this long chain conditional! I often think of this question as an excellent example of conditional reasoning that may not appear to make much sense in the "real world." As in, "Hey, what does it matter what kind of listeners are in the audience? Why couldn't the musicians do an inspired job either way?"

But of course we're not dealing with the real world; we're dealing with the LSAT.

Again, I hate to split the difference here, but I don't see much downside to either approach. Personally, I habitually indicate the contrapositives as I go, especially when there are compound sufficient or necessary statements that need to be negated. On the other hand, this approach can lead to a long, redundant list of conditionals and contrapositives.

Thus, this becomes more of a stylistic question of personal preference. If you're more comfortable jotting the conditionals down first and then linking them up both ways, go for it! If you're more comfortable doing the contraposition for each conditional first, then do that. As I noted, there may be circumstances that prompt you to do more spot analysis, as in when you're dealing with difficult or compound conditionals.

Thanks for the excellent questions!